News Release

New Images Reveal Different Magma Pools Form the Ocean's Crust

For the first time, scientists have produced images of the oceanic crust
and found that the upper and lower layers of the crust are likely
formed from different magma pools. The images begin to answer
some lingering questions about where new ocean crust comes from and
whether it is all formed the same way.

Geophysicists Robert Detrick and Juan-Pablo Canales of Woods Hole
Oceanographic Institution (WHOI) and colleagues used reflected seismic,
or sound, waves to successfully image the structure of the lower crust
across the flanks of the Juan de Fuca Ridge, a spreading plate boundary off the
Pacific Northwest coast. Their study, co-authored by researchers at
Columbia University’s Lamont-Doherty Earth Observatory and Scripps
Institution of Oceanography, appears in the August 25, 2005 issue of Nature.

By recording the reflection of seismic waves off the lower crust at the crust-mantle
boundary, a technique
common in oil exploration, the researchers found evidence strongly suggesting
that the base of the crust forms much differently than its overlying
layers.

“Seismic reflection is a powerful tool to image the sub-surface
detailed structure of the Earth down to several kilometers or miles
below the surface," study
co-author Canales said. “Scientists studying the formation of the ocean
crust have been debating over the past decade whether all of the crust
is formed from magma that accumulates in a single pool or lens a mile
or two
deep, or if it forms from multiple magma sills at different levels.”

Detrick, Canales and colleagues analyzed about 1,500 kilometers (935
miles) of data collected on the Juan de Fuca Ridge off the coast of
Washington, Oregon and northern California. The images are the
first of their kind showing solidified magma lenses and sills, narrow
lateral intrusions of magma, embedded in the boundary between the mantle and the
overlying crust, a region known as the Moho transition zone. The
existence of these magma lenses near a mid-ocean ridge suggests that the
lower oceanic crust is formed from several smaller sources of magma
rather than a single large pool located in the middle of the crust.

Unlike continental crust, which is very old and thick, oceanic crust averages 6-7
kilometers (3-4 miles) thick and is constantly being recycled at
tectonic plate boundaries on the seafloor. Crust is destroyed at
subduction zones, where plates come together, and created at mid-ocean
ridges, where plates are pulling apart, like the Juan de Fuca Ridge. At
these ridges, also known as seafloor spreading centers, molten rock, or
magma, rises from deep within the earth and solidifies to become new
crust. But the exact source of that magmaparticularly the magma that
forms the lower layers of the crustwas not well understood until now.

Previously, geophysicists knew that the topmost layer of the crust
cooled from molten rock supplied by a single pool, or lens, of magma
located in the crust’s middle layers. What was not known was whether
the lower crust, which lies just above the mantle, solidified from the
same melt lens or from many smaller magma bodies in the deeper
crust-mantle transition zone. The new study found evidence of multiple
pockets of molten rock now frozen, lending strong support to the latter theory.

Geophysical studies along mid-ocean ridges to date using seismic
reflection have been able to image only one single crustal melt lens,
supporting the first model of crustal formation. However, other
remote-sensing geophysical methods that are used to infer the mechanical properties of the crust indicate that magma must
also accumulate at deeper levels, in particular at the base of the
crust or the Moho transition zone.

The multiple-lens model comes from field observations at
ophiolites where the remnants of the multiple melt sills can be
mapped. Ophiolites are slabs of oceanic crust long ago thrust up
onto dry land and are easily accessible to geologists seeking clues to
what new crust might look like.

“It is exciting that different observational approaches, marine
seismology and ophiolite studies, that look at the same problem at
different spatial and resolution scales are converging towards a
unified geological and geophysical model of how the ocean crust is
formed," Canales said.

The study was funded by the National Science Foundation.

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Originally published: August 25, 2005

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